A new study confirms what many restaurateurs have long suspected: The number of customers who actually have a food allergy is a small portion of those who claim to have one.

The report, published this week in the Journal of the American Medical Association, found that 19% of American adults believe they are allergic to one or more foods. But only about half, or 10.8%, actually have an intolerance, the study found.

But the report also underscores the danger of exposing someone with an actual food allergy to whatever sets them off. More than a third (38%) of the true allergy sufferers had to visit an emergency room because they encountered the trigger ingredient.

Even with wrong self-diagnoses, the number of allergy sufferers in the United States is huge—more than 26 million adults, according to the findings.

The most common allergies, in descending order, were shellfish, milk, peanuts, tree nuts and finned fish. Nearly half (45.3%) of the true allergy sufferers had a sensitivity to two or more foods.

The milestone report also shows that allergies can develop at any point in a sufferer’s life. Nearly half the respondents (48%) who were adjudged to have a true food allergy did not develop the intolerance until adulthood.

“These findings suggest that it is crucial that adults with suspected food allergy receive appropriate confirmatory testing and counseling to ensure food is not unnecessarily avoided,” reads the report.

The data is based on surveys of 40,443 adults between October 2015 and September 2016. The study was conducted by Ruchi S. Gupta, a medical doctor; Christopher Warren; and Bridget Smith. All three are associated with medical facilities in the Chicago area.

A synopsis of the report, “Prevalence and Severity of Food Allergies Among US Adults,” can be found here.